1. Field of the Invention
The present disclosure relates to additive manufacturing devices and methods and more specifically to build platforms for use in such devices.
2. Description of the Related Art
Additive manufacturing or 3D printing refers to machines and methods for fabricating parts layer by layer in an additive manner instead of in a subtractive manner, which tends to waste raw material, take additional time, and limits the geometry of the design. One method of producing parts by additive manufacturing, known as Fused Deposition Modelling (FDM), is by using a heated nozzle to extrude a thermoplastic material to create each 2D layer of a part on a build platform. The thermoplastic material may be in filament or pelletized form and may be blended with fibers for additional strength and dimensional stability.
A part is fabricated by depositing a base layer of material with a nozzle on the build platform to define the perimeter shape and any internal features of the layer. Each additional layer is deposited on the previous layer to define that section of the part. At the completion of each layer, either the nozzle raises upward or the platform lowers downward an amount approximately equal to the thickness of the next layer. When the additive process is complete, the 3D part is removed from the build platform and finished according to specification requirements. Part removal and machine initialization for the next part is generally accomplished by a human operator, which can limit throughput and increase part cost.
Currently in FDM, the bond between the FDM part and the platform it is built on relies on an adhesive bond, which can be unreliable. During the build, the thermoplastic material cools and shrinks, breaking the bond between the first layer of the print and the build platform. This breaking of bonds in conjunction with the forces from the extrusion nozzle at the top of the part causes delamination of the part during the build, which requires the build to be restarted from the beginning. The failure of FDM builds due to delamination is the major reason for FDM build failure and causes a great amount of wasted machine time and material.
Some extrusion type additive manufacturing machines include a heated build chamber and/or a heated build platform to reduce material shrinkage and the resulting quality defects such as delamination, curling, lifting, warping and bending. While a heated build chamber may be practical for chambers with build envelopes in the range of 24 inch (610 mm) or even 36 inch (914 mm) on a side, build envelopes in the range of 10 feet (3.05 meter) or even 40 feet (12.19 meter) on a side are much more impractical and costly to uniformly heat. Fabricating parts without a heated chamber and without having the above-mentioned quality defects can be a challenge. Furthermore, machines with heated build chambers utilize textured build platforms that are designed to optimize adhesive forces, but these platforms are not reusable and add to the cost and waste of the process.
Moreover, unintentional movement of a partially-completed part may cause further quality defects. In one example, a part may acquire a noticeable step or shift in the exterior surfaces at a certain level, and, in another example, the part may actually tip over while additional material continues to be extruded. Adhesion between the part and the build platform can be marginally improved with: the use of bonding adhesives; the application of double sided, adhesive tape; and the roughening or texturing of build platforms to improve adhesion as described in U.S. Pat. No. 8,226,395.
Big Area Additive Manufacturing (BAAM) systems as described in U.S. patent application Ser. No. 14/143,989 are capable of generating parts weighing up to 500 pounds (226.8 Kg) or even up to 2000 pounds (907 Kg) or more for the automotive, aircraft and tooling industries. For example, the Strati electric car, which was manufactured by Local Motors in collaboration with Cincinnati Incorporated and Oak Ridge National Laboratory at the 2014 International Manufacturing Technology Show in Chicago, USA, weighs nearly 2000 pounds (907 Kg).
Large scale parts such as the Strati electric car often undergo secondary operations such as: CNC machining of exterior surfaces; CNC boring of holes, slots, and other features; coating and plating; as well as other finishing operations. In order to properly fixture the part in the various secondary machines for post processing, it is advantageous to have the part remain on the build platform to maintain the original coordinate system that was used to build the part. Secondary finishing operations can induce significant forces on the part, which can cause the adhesion forces between the part and the build platform to fail, thus compromising the original coordinate system.
What is needed are additive manufacturing build platforms that mechanically engage a part for improved part quality, reduced human intervention and increased throughput.